1. Field of the Invention
The present invention relates to a liquid ejection head having a plurality of nozzles.
2. Description of the Related Art
In a recording apparatus equipped with an on-demand liquid ejection head that ejects ink only during recording, ink is ejected in the form of a droplet from a minute opening for ink ejection (hereinafter referred to as “ejection orifice”) provided at one end of each nozzle. At that time, according to the amount of liquid forming the droplet, the meniscus formed in the nozzle moves back. After that, the meniscus is pulled back to the ejection orifice by capillary action. After that, the filled state of the nozzle returns to the state before the ejection. Such a phenomenon is called refill.
In general, in order to increase the recording speed of an on-demand ink jet recording apparatus, the drive frequency is increased, or many nozzles (ejection orifices) are provided in one liquid ejection head. A thermal ink jet recording apparatus, which is one of on-demand ink jet recording apparatuses, has a liquid ejection head that has a simple structure and in which nozzles can be easily arrayed at high density. For this reason, in thermal ink jet recording apparatuses, the recording speed is increased by integrally forming many nozzles.
FIG. 18 is a waveform diagram showing the refill behavior in the case where ink is ejected from a single nozzle and the refill behavior in the case where ink is ejected from many nozzles. In FIG. 18, the horizontal axis shows elapsed time since the ink ejection, and the vertical axis shows the amount of displacement of the meniscus after the ink ejection. On the vertical axis, the position coplanar with the ejection orifice (hereinafter referred to as “ejection orifice plane”) is zero (reference). When the amount of displacement is positive, the meniscus is bulging from the ejection orifice plane. When the amount of displacement is negative, the meniscus is displaced from the ejection orifice plane into the nozzle. In FIG. 18, the waveform 101 shows the refill behavior in the case where ink is ejected from a single nozzle, and the waveform 106 shows the refill behavior of a typical nozzle in the case where ink is ejected from many nozzles.
As shown in FIG. 18, after the ink ejection, the meniscus bulges from the ejection orifice plane. After that, the meniscus shows behavior like damped vibration about the ejection orifice plane. In this specification, the time from the ink ejection until the amount of displacement of the meniscus first returns to zero will be referred to as “refill time.”
In a thermal liquid ejection head, in the case where ink is ejected from many nozzles at the same time or at a slight interval, the pressure waves of ink generated at the time of bubble formation propagate to a common liquid chamber communicating with each nozzle. For this reason, the sum of the pressures propagating from each nozzle to the common liquid chamber becomes a large force acting in the direction opposite to the direction of refill in each nozzle. As a result, in the case where ink is ejected from many nozzles, the refill time t6 of each nozzle is long compared to the refill time t1 in the case where ink is ejected from a single nozzle. As shown in FIG. 18, in the case where ink is ejected from many nozzles, the amplitude A6 of the meniscus is large compared to the amplitude A1 in the case where ink is ejected from a single nozzle. For this reason, it takes long time before the meniscus returns to a stable state where the amount of displacement of the meniscus is zero. If the next ejection of ink is performed with the meniscus in an unstable state, ejection failure may occur, for example, the amount of liquid forming an ink droplet may change, or the accuracy of the ink ejecting direction may be deteriorated. Such ejection failure may cause a decrease in recording quality due to the change in the diameter of ink dots formed on a recording medium, or blurs, streaks, missing dots, or the like in a recorded image due to a decrease in the landing accuracy of ink droplets onto a recording medium. For this reason, the drive frequency needs to be set within a range where the next ejection is not performed when refill is unstable. As a result, if it takes long time before the meniscus returns to a stable state, it is difficult to increase the drive frequency. Therefore, the increase in the amplitude of the meniscus prevents increasing the number of nozzles.
Japanese Patent Laid-Open No. 7-156403 discloses one of the methods to solve the problems of the long refill time and the large amplitude of the meniscus in the case where ink is ejected from many nozzles. Japanese Patent Laid-Open No. 7-156403 discloses a method including trapping a bubble at an arbitrary position in the common liquid chamber and absorbing the pressure change in the common liquid chamber with the bubble.
In the case where a bubble is used as a pressure buffer like the liquid ejection head described in Japanese Patent Laid-Open No. 7-156403, the capacity C showing the compressibility of a bubble existing in the common liquid chamber is given by the following Equation (1):
                    C        =                              V            bub                                P            bub                                              Equation        ⁢                                  ⁢                  (          1          )                    where Vbub is the volume of the bubble, and Pbub is the pressure of the bubble. The pressure buffering effect of a bubble changes depending on the capacity C.
In the case where a bubble is used as a pressure buffer, it is very difficult to maintain the shape (volume) of the bubble constant for a long time in the common liquid chamber. The volume of the bubble decreases over time, and the capacity C also decreases from the above Equation (1). Therefore, the pressure buffering effect in the common liquid chamber decreases. Therefore, in the case where a bubble is used as a pressure buffer, the pressure buffering effect in the common liquid chamber cannot be maintained, and therefore, it is difficult to stabilize high-speed recording.